Linky(Coincidence here, I went to UCL for my Master’s. Great environment, great teachers. A year I genuinely treasure.)

Primatologists are supposed to be obsessed with dominance heirarchies, even half-trained ones like me. This particular issue isn’t just about dominance but also about eusociality, which is when most adults in a given group don’t try to reproduce. At its most elaborate level we see this in ants, bees, and wasps (all closely related), but we also see some degree of this among many social mammals, including naked mole rats (the hallmark example), wolves, and marmosets and tamarins (the guys I study). Since the end goal of all organisms is to make babies, one would think that this is a pretty odd arrangement. In certain contexts it can make a lot of sense, though, depending on associated tradeoffs.

In 1964, W.D. Hamilton solved a crucial part of this enigma when he proposed the idea of relative fitness. Traditionally, fitness has been defined as how many babies you make that live to reproductive age. However, Hamilton realized that the only difference between one’s offspring and one’s relatives is the strength of genetic similarity. You share 50% of your genes with each of your parents, your siblings, or your children. 25% with a first cousin. Rather than bore you with the details, lets just leave it with the conceptual framework that the more closely related you are to an individual, the more you gain by helping them out (in other words, playing ‘wingman’ for your brother is as worthy of effort as raising your own children). Although the issue could get a lot thornier (for instance although your parents share the same number of genes with you as your children do, your parents are a lot less likely to have babies, so in theory you should be less likely to help them), we’ll leave it at that for this discussion.

Scientists at UCL (University College London) have discovered that even wasps are driven by their status. The study, published today in Nature, shows that lower-ranked female wasps work harder to help their queen than those higher up the chain because they have less to lose, and consequently are prepared to take more risks and wear themselves out.

Because they’re eusocial, all of the female wasps tend to be sisters or half-sisters (I forget the mechanics of it, which are unimportant for the discussion anyway). So they get quite a bit of benefit from helping the one who actually does the breeding in raising the babies. But here we see the tradeoffs that complicate the issue. The higher up in the heirarchy you are, the more likely it is that you’ll get a turn as the breeding female. Those with a looong succession ahead of them have little or no chance at ever breeding. Too many queens would have to die. Think of the 3rd or 4th son of a king taking a military career where he has a much higher likelihood of dying than his layabout older brothers (who even if they don’t get kingship at least will get a decent parcel of property).

To quote the study’s author, Dr. Jeremy Field:

The wasps in this queue face a fundamental trade-off: by working harder, they help the group as a whole and as a result indirectly benefit themselves, but they simultaneously decrease their own future survival and fecundity because helping is costly. It involves energy-expensive flight to forage for food, and leaving the nest is dangerous. We have found that the brighter the individual wasp’s future, the less likely it is to take risks by leaving the safety of its nest to forage for food.”

In other words we’re seeing what amounts to a cost/benefit analysis in each of these females. The older you are, the more cost there is to helping and the less relative benefit. For an illustration, half of you imagine you’re female. The other half stay as you were. Now imagine you have two sisters, one who’s fertile, and one who’s barren. The fertile one is still searching for a sperm donor, but she’s hot and wears skimpy clothing, so there’s little doubt she’ll find one. Your husband tragically dies during childbirth from blunt force trauma as you knock his head against the wall screaming “WHY DID YOU DO THIS TO ME YOU BASTARD!” And on top of that you just had triplets. Which sister is more likely to help you out? The barren one. If she stays at home Saturday nights, it’s not like she loses any reproductive opportunity. On the other hand, the one with the fully functioning ovaries would lose major mating opportunities if she were to give up her nights and weekends during these, the most fecund and attractive years of her life. The lower-ranking wasp females might as well not have ovipositors, their likelihood of breeding is so low.

Now, because this is so close to a topic I spend inordinate amounts of time on, I’m going to contrast this situation to that which you see in Marmosets and Tamarins. Most monkeys mothers raise the kids on their own while Dad spends all his time eating, scratching himself in less than genteel places, showing off his canines, grunting, beating up every other male that looks at him funny, and trying to copulate with anything that moves and is suitably equipped. When it comes to certain South American monkeys, instead what we see are mothers that are basically walking incubators and milk dispensers. Very similar to queen wasps in that respect except that I don’t think you can milk a wasp (and at around half a pound, it’s pretty hard to milk a marmoset too. Trust me…I’ve tried). This is because they routinely produce twins, which is hard to manage considering monkey infants are already pretty expensive to take care of compared to other mammal babies. Not only does dad help out, but the infant’s older adult siblings stick around and help as well.

With these guys, there are a couple of reasons why they’d do this instead of going off to pitch woo with similarly young and free adult offspring from neighboring troops. First, marmosets need territory (which makes the wooing situation a lot like the Montagues and Capulets). It’s hard to maintain territory when it’s just a male and a female. Not to mention that your population is high enough that you’re completely surrounded by other marmoset groups. So these guys bide their time waiting for an opening. But there’s another aspect as well. Parenting is not necessarily natural for males. But they’re the ones who do most of the behavorial parenting (carrying, provisioning, playing, protecting, watching) as compared to the physiological stuff (pregnancy and lactating). So older sons (and daughters to an extent) can get a signifiant benefit by ‘training’ with a younger sibling. They do better when they finally leave and have higher success rates with their babies. In other words, while a lower ranking wasp is more likely to help out, in marmosets you see the reverse. Because the older you are, the more likely you are to be able to get in on new territory (dominance and preference), have a mating opportunity, and need the ability to parent. The younger you are, the less likely you’ll be making babies any time soon, so the less reason you’d have to learn how to do it (and in the process help out mom and dad).

While not earth-shattering, this is just a cool find in that it continues to expand our knowledge, understanding, and appreciation of how the many factors and variables involved in reproduction and survival play into the choices an animal makes, and in the determination of a given social system. It is something all too easy to miss, whether one is a geneticist blathering on and on about genes being the only unit of selection (some geneticists are pretty cool, though. I’ve been having a lot of fun going back and forth with Gene Expression recently…and his linking me has improved my stats) or one of those silly humanists who can only think of nature as ‘red in tooth and claw’. The ways in which self-interested behavior can result in such complex, cooperative societies is a thing of beauty unparalleled, like a snowflake growing in size and complexity at the behest of ever-so-simple a rule.